Copolymer-type pigment dispersing agents have been developed in a variety of fields. In the field of, for example, color liquid crystal displays, a copolymer-type pigment dispersing agent is used in producing a color filter by a pigment dispersion method. A color filter is required to show performances of high contrast, high luminance, and the like. A pigment that has a peculiar transmission absorption spectrum in the visible light wavelength region according with the fluorescence emission spectrum of a backlight is suitably used. In order to increase the contrast, it is significant to minimize the light scattering in a color filter layer, and hence, a pigment made finer to have a primary particle size of several tens nm or less is used.
In order to attain high luminance of a green picture element, a combination of a green pigment and various yellow pigments has been conventionally employed. A pigment in the form of finer particles is, however, easily aggregated again, and in addition, a pigment obtained as the combination of a green pigment and various yellow pigments is liable to be unstable in a dispersion system, and thus, there was no suitable dispersing agent.
Patent Document 1 proposes a dispersing agent to be used in the case where a brominated zinc phthalocyanine green pigment is used for purpose of increasing the luminance of a green picture element. This document proposes a dispersing agent that is a copolymer of a block A having affinity for a solvent and a block B having a functional group comprising a nitrogen atom, and has an amine value, in terms of effective solid content, of 80 mgKOH/g or more and 150 mgKOH/g or less.
A color filter described in this document is, however, insufficient in heat resistance and the like of a green picture element.